The present invention relates in general to crystal mounting arrangements for quartz crystal packages and more particularly to a low-profile crystal package with an improved crystal mounting arrangement which facilitates multiple electrical connections to each surface of the crystal plate without flexing or repositioning the crystal plate.
It has long been the practice to hermetically seal the protective metal crystal package. Once sealed, the crystal package can be used in many different environments with a high degree of relative confidence that the internal crystal plate will perform at the desired frequency.
In recent years, space limitations have required smaller and smaller crystal mounting assemblies, which has caused an increase in the electrical and mechanical problems relative to adequately fixturing a crystal plate within a crystal package, while not causing damage to the crystal plate, maintaining frequency standardization and avoiding internal environmental contamination during the sealing process. Likewise, as crystal applications have become more and more complex, more electrical contact points are required, on smaller and smaller crystal plates. Thus, it has been the prior-art practice to provide a small crystal package which allows the mounting of the crystal plate within a metal evacuated container, such container having terminal pins projecting within the internal cavity which are connected via wire leads to the electrodes found on the crystal plate. These same terminal pins also serve to support the unit within the evacuated cavity as its seams are sealed either by soldering, hot-welding, or cold-welding technologies.
The use of high-pressure cold-welding technology has found favor in the prior art since it has the primary advantage of being free from fumes and contamination; due to the pressures involved, it has an occasional disadvantage of causing distortion to the metal container during the welding operation, which may result in the internal terminal pins being displaced from their intended positions. Since the crystal plate is secured by the terminal pins, will cause a stress force to be applied to the crystal plate, which will have negative consequences to the frequency characteristics of the crystal plate. The present invention is concerned with a crystal package that is preferably hermetically sealed by the utilization of cold welding apparatus, but is not limited to solely the cold welding technological art.
In the prior art there are various types of crystal mounting apparatus which are in common usage. In one approach, the quartz crystal wafer is supported by the center mounting post, which allows the horizontal mounting of the crystal and reduces the overall height of the crystal package assembly. A disadvantage of this crystal mounting approach is that the crystal plate needs to be adequately supported during the wire bonding process so as to withstand the surface pressures involved without sustaining damage to the crystal plate. During assembly of the crystal wafer onto its respective center post mounting position, it is desirable to avoid overstressing the wafer. Such stresses can occur by the means which are used to mount the crystal wafer in the crystal package or by the mechanical stresses which develop during the hermetic sealing of the crystal package itself.
The prior art sealing techniques have generally consisted of the steps of hermetically sealing a pair of terminals in an eyelet to form a header, mounting a crystal wafer on an associated header, and hermetically sealing the header to a metal or glass container, with the crystal wafer positioned within such container. Headers have been characterized either as matched glass or compression glass headers. In a glass header, a single vitreous material is used to fabricate the terminals and eyelets, while a second vitreous material which has the same thermal co-efficient of expansion as the eyelet and terminals is used to seal the terminals to the eyelet. Generally, the eyelet and terminals are made of an iron-nickel-cobalt alloy material such as KOVAR and the vitreous material is a glass which has been selected to have a thermal co-efficient expansion substantially the same as that of KOVAR. The sealing process is facilitated by placing molten glass in the eyelet around the terminals and then cooling the glass to cause it to shrink and tightly grip the terminals. The similar coefficients of thermal expansion of the KOVAR components and the vitreous material insure that the seal between the terminals and the eyelet will be maintained. In both the matched and the compression glass headers the integrity of the terminal to eyelet seal depends upon the attainment of a good seal between the terminal and the glass.
A disadvantage common to all of the prior-art devices is the relatively high cost and complexity which exists for the various crystal package apparatus.